<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div{
      margin: 30px;
      border: 5px solid #000;
      width: 200px;
      height: 100px;
      padding: 20px;
      /* box-sizing: border-box;  IE盒模型 */
      /* box-sizing: content-box;  标准盒模型 */
    }
  </style>
</head>
<body>
  <div>hello world</div>
</body>
</html>